The land area and amount of pipe needed in a horizontal GHX is directly related to the thermal properties of the soil it is buried in. This in turn relates directly to the cost of installing a GCHP system...and if your customer feels it's too expensive to install they typically resort to installing a conventional system and forget about the heat pump system. To make it worse, that potential customer will often tell his or her friends they considered it for their home, but it was too expensive...they'd never get their money back. Is there any way of avoiding that?
One of the most effective ways to avoid the uncertainties of sizing the GHX is good analysis of the project. Accurate building heating and cooling loads are first and foremost. After the building loads, it's critical to know the thermal properties of the soil and rock you are building the GHX in. If you are considering building a horizontal GHX, you can estimate the thermal conductivity and diffusivity of the soil, play it safe and add a safety factor, or you could accurately measure the properties of the soil with a probe meter in just a few minutes. Decagon Devices builds a meter that accurately measures the:
Rather than guessing at these numbers based on tables and graphs, you can measure the soil properties in a matter of minutes as long as you can insert the instrument probes into the soil of a fresh excavation. Or you can bring back a consolidated soil sample and measure the properties in your office.
You can also use the device to measure the properties of the grout you use in a vertical borehole to make sure you are getting the heat transfer specified...in the field or in the lab. This device is a very useful tool to ensure your GHX will perform as you've designed it...first with a detailed and accurate heating and cooling load of the building, and secondly by accurately measuring the thermal properties of the soil you're building it in.
In my blog I'll be expressing my opinions about what I've the learned about ground coupled heat pump (GCHP) systems over the last 30 years. I've been very fortunate to work with many interesting people who are passionate about this technology...engineers, geologists, mechanical contractors, drillers, excavation contractors...in different parts of the world. I've learned a lot from them and will be using this forum to pass on some of the things I've learned and feel are important. Please feel free to use this information if you feel it's worthwhile...hopefully you can avoid some of the same mistakes I've learned from.